A prolonged and intense heat wave has begun to swelter much of the Southwest this week — and by early next week, dozens of records are likely to have been set.

What to expect: The heat wave is forecast to culminate with a dangerously hot Labor Day weekend across much of California, Nevada, Oregon and surrounding states in the West.

Threat level: In Death Valley, California, which saw flash floods earlier this summer, September heat records could be threatened or toppled, with a high at or above 125°F this weekend.

• The San Diego, Sacramento and Los Angeles areas could also reach record-high heat levels this week, per the National Weather Service.
• "Widespread high to very high heat risk is anticipated across interior NorCal, impacting the entire population," the agency said in an online forecast discussion.
• Reno, Nevada, is also expecting an extended period of unseasonable heat, warming to near-record temperatures by midweek, the NWS said.

The big picture: This heat wave is likely to break daily, monthly and even all-time records and is forecast to do so for several days in a row across a large region.

• Wildfire risks are already climbing in several states, with red flag warnings in effect in California, Oregon and Washington.

By the numbers: Nearly 31 million people are now under excessive heat warnings, according to NWS.

• Another 11 million are under heat advisories, with 14 million under excessive heat watches.

Why it matters: Extreme heat is a major public health threat, increasingly taking a toll on children, pregnant people, the elderly and other vulnerable populations, and making summers deadlier.

• Some cities like Los Angeles and Sacramento have opened cooling shelters for residents to find refuge from the heat.

Our thought bubble: The culprit for the heat wave is a large heat dome, or area of high pressure, building over the region and growing more intense with time. Even states that saw beneficial monsoon rains, such as Arizona and Nevada, are under heat warnings.

• Extreme heat events such as this one are becoming more common and intense due to human-caused climate change. This is just the latest of several heat waves to hit the Southwest this summer.

What they're saying: "We are feeling the heat today, but the worse is yet to come with an extended heat wave through at least early next week," the NWS office in Los Angeles said Tuesday.

• "This will result in very dangerous heat given 1) extreme afternoon highs, 2) oppressive overnight lows and 3) long duration heat," the NWS forecast office in Sacramento said in an online forecast discussion.

Go deeper: An "Extreme Heat Belt" will soon emerge in the U.S., study warns